This invention relates to a process for preparing aqueous dispersions of rosin-base materials, and more particularly to a process for preparing aqueous dispersed size of enhanced alum sensitivity and lowered tendency to foam.
Cellulose fiber products such as paper and paperboards are produced from an aqueous slurry, or furnish, of cellulose fibers containing sizing agents admixed therewith. These sizing agents generally comprise aqueous dispersions of rosin, especially fortified rosin, which is utilized to modify the surface of the paper to control water penetration. Such sizing is termed internal sizing and is an important step in the wet end operation of a paper machine.
Rosin, or rosin acid, itself has no affinity for cellulose fiber and must be anchored to the surface of the cellulose fiber with a cation such as an aluminum ion normally derived from alum. The rosin acid size and aluminum ions do not react in solution but are codeposited on the fiber surface. The size is held on the pulp fiber by electrostatic forces. This rosin acid size is not yet hydrophobic, but it becomes water-repellent after interaction with the alum in a subsequent heat curing step. The curing occurs when rosin size melts in the drier section of the paper machine and the molten rosin acid spreads over the fiber surface and reacts with neighboring alum adsorbed on it. The resulting aluminum-rosinate is to a large extent responsible for the degree of water repellency of the paper product.
Attention is directed to the disclosures of related U.S. Pat. Nos. 4,267,099 and 4,309,338 to Okumichi et al for an in depth discussion of a method of preparing a dispersed rosin size by the inversion method, which patents are incorporated by reference herein.
Okumichi et al provide a process for preparing an aqueous dispersion of a rosin-base material by the inversion method characterized by reduced foaming properties achieved by use of at least one of the dispersants disclosed and claimed therein. While dispersed rosin size prepared in accordance with Okumichi et al and particularly sizes produced with a dispersant selected from the salts of sulfuric acid half ester of formula II of U.S. Pat. No. 4,267,099, referred to by Okumichi et al as "sulfates," provides dispersed rosin size of reduced foaming properties, the size still tends to produce excessive foam under conditions normally encountered in some papermaking machines.
Okumichi et al thus approached the problem of lowering the tendency of a dispersed rosin size to foam by specifically tailoring the dispersant, or surfactant, albeit they do recognize the obvious expedient of lowering the surfactant level in the size to lower the tendency of the size to foam. Unfortunately, a simple lowering of surfactant level is not practicable because surfactant level is tied to the very ability to produce a dispersion.
Attempts to lower foam generation with dispersed rosin size such as disclosed by Okumichi et al and having a lowered level of the salts of the sulfuric acid half ester of formula II of U.S. Pat. No. 4,267,099 to Okumichi et al, even with the addition of cosurfactants has generally not met with success. Along the same lines, the effect of increased rosin fortification was studied and no appreciable effect on foam generation was noted. However, a detrimental effect on size particle dimension was noted, i.e. the particle size was increased wherein desirable product qualities were compromised without appreciable decrease in foam.
Kawatani et al, in Japanese Kokai No. 79 58, 759, provide another approach to lowering the tendency of aqueous rosin dispersions to foam through the use of internal foam depressors. Kawatani et al teach the use of simple aliphatic acids, e.g., caproic, caprylic, lauric, or myristic, for this purpose. This method of foam lowering is unappealing because the amount of rosin available for sizing is reduced, contaminates with unknown effects are introduced, and the basic problem of inefficient surfactant usage is neglected.
It is known that rosin itself has no affinity for cellulose fiber and is generally anchored to the surface of fiber by utilization of alum. Between pH 4.7 and 5.0, complex polymolecular forms of aluminum ions have been found to be prevalent in a paper making furnish containing alum. This complex Al.sub.8 (OH).sub.20.sup.4+ is highly charged and its OH groups can easily interact with the COOH groups from rosin or cellulose. Adsorption of aluminum on fiber rapidly increases in the pH region where the aluminum complex species is formed.
Thus, a need still exists for a dispersed rosin size of reduced foaming tendency and enhanced alum sensitivity.